Navigational guidance via computer-assisted fluoroscopic imaging

ABSTRACT

Digital x-ray images taken before a surgical procedure by a fluoroscopic C-arm imager are displayed by a computer and overlaid with graphical representations of instruments be used in the operating room. The graphical representations are updated in real-time to correspond to movement of the instruments in the operating room. A number of different techniques are described that aid the physician in planning and carrying out the surgical procedure.

RELATED APPLICATIONS

This disclosure is related to U.S. patent application Ser. No.09/106,109, entitled “System and Methods for the Reduction andElimination of Image Artifacts in the Calibration of X-Ray Imagers,”filed on Jun. 29, 1998.

FIELD OF THE INVENTION

The present invention is directed generally to image guided surgery, andmore particularly, to systems and methods for using one or morefluoroscopic X-ray images to assist in instiuent navigation duringsurgery.

DESCRIPTION OF THE RELATED ART

Modern diagnostic medicine has benefitted significantly from radiology,which is the use of radiation, such as x-rays, to generate images ofinternal body structures. In general, to create an x-ray image, x-raybeams are passed through the body and absorbed, in varying amounts, bytissues in the body. An x-ray image is created based on the relativedifferences in the transmitted x-ray intensities.

Techniques are known through which x-ray images are used to locate thereal-time position of surgical instruments in the patient anatomyrepresented by the x-ray image without requiring x-rays to becontinually taken. In one such system, as disclosed in U.S. Pat. No.5,772,594 to Barrick, light emitting diodes (LEDs) are placed on a C-armfluoroscope x-ray imager, on drill, and on a reference bar positioned onthe bone to be studied. A three-dimensional optical digitizer senses theposition of the LEDs, and hence the position of the drill, the C-armfluoroscope, and the object bone. Based on this information, thereal-time position of the drill in anatomy represented by the x-rayimage is determined, and a corresponding representation of the drill inthe x-ray image is displayed. This allows the surgeon to continuallyobserve the progress of the surgery without necessitating additionalx-ray images.

Surgical navigational guidance, as discussed above, can provide a toolfor helping the physician perform surgery. It is an object ofthe presentinvention to provide several enhancements to traditional surgicalnavigational guidance techniques.

SUMMARY OF THE INVENTION

Objects and advantages of the invention will be set forth in part in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention will be realized and attained by meansof the elements and combinations particularly pointed out in theappended claims.

One aspect of the present invention is directed to an x-ray imagingdevice comprising a plurality of elements. In particular, the x-rayimaging device includes an x-ray source for generating cycles of x-rayradiation corresponding to an image acquisition cycle; an x-rayreceiving section positioned so that x-rays emanating from the x-raysource enter the x-ray receiving section, the x-ray receiving sectiongenerating an image representing intensities of the x-rays entering thex-ray receiving section. Additionally, a computer is coupled to thex-ray receiving section and radiation sensors are located in a path ofthe x-rays emitted from the x-ray source. The radiation sensors detectthe beginning and end of a radiation cycle and transmit the detectedbeginning and end of the radiation cycle to the computer.

Another imaging device consistent with the present invention includes arotatable (C-arm support having first and second ends. The first endincludes an x-ray source for initiating an imaging cycle and the secondend includes an x-ray receiving section positioned so that x-raysemanating from the x-ray source enter the x-ray receiving section. Thex-ray receiving section generates an image representing the intensitiesof the x-rays entering the x-ray receiving section. Further, acalibration and tracking target is included and a tracking sensordetects the position, in three-dimensional space, of the calibration andtracking target; and a computer is couples to the x-ray receivingsection and the tracking sensor. The computer detects motion of theC-arm based on changes in the position detected by the tracking sensor.

Another aspect consistent with the present invention is directed to asurgical instrument navigation system. The system comprises a computerprocessor; a tracking sensor for sensing three-dimensional positioninformation of a surgical instrument and transmitting the positioninformation to the computer processor; a memory coupled to the computerprocessor, the memory including computer instructions that when executedby the computer processor cause the processor to generate an iconrepresenting the surgical instrument and to overlay the icon on apre-acquired x-ray image, the icon of the surgical instrumentrepresenting the real-time position of the surgical instrument projectedinto the pre-acquired x-ray image and the icon being generated as firstrepresentation when the surgical instrument is positioned such that itis substantially viewable in the plane of the pre-acquired image and theicon being generated as a second representation when the surgicalinstrument is positioned such that it is substantially perpendicular tothe plane of the pre-acquired image. Finally, a display is coupled tothe processor for displaying the generate icon superimposed on thepre-acquired image.

Yet another system consistent with the present invention comprises acomputer processor and a tracking sensor for sensing three-dimensionalposition information of a surgical instrument and transmitting theposition information to the computer processor. A memory is coupled tothe computer processor, the memory including computer instructions thatwhen executed by the computer processor cause the processor to generatean icon representing the surgical instrument positioned in apre-acquired image of a patient's anatomy, the icon of the surgicalinstrument including a first portion corresponding to an actual positionof the surgical instrument and a second portion corresponding to aprojection of the surgical instrument along a line given by a currenttrajectory of the surgical instrument. A display is coupled to theprocessor for displaying the generated icon superimposed on thepre-acquired image.

Still further, another surgical instrument navigation system consistentwith the present invention comprises a rotatable C-arm including anx-ray source and an x-ray receiving section for acquiring x-ray imagesof a patient, the C-arm being rotatable about one of a plurality ofmechanical axes. A computer processor is coupled to the rotatable C-armand a memory is coupled to the computer processor. The memory stores thex-ray images acquired by the rotatable C-arm and computer instructionsthat when executed by the computer processor cause the computerprocessor to generate a line representing a projection of a planeparallel to one of the plurality of the mechanical axes of the C-arminto the x-ray image, the line enabling visual alignment of the one ofthe plurality of mechanical axes of the C-arm with an axis relatingcomplimentary image views. A display is coupled to the processor fordisplaying the generated line superimposed on the x-ray image.

Yet another system consistent with the present invention is for defininga surgical plan and comprises an x-ray imaging device; a surgicalinstrument; a tracking sensor for detecting the position, inthree-dimensional space, of the surgical instrument; a computerprocessor or in communication with the tracking sensor for defining apoint in a virtual x-ray imaging path is the three-dimensional locationof the surgical instrument, the point being outside of a true x-rayimaging path of the x-ray imaging device, the computer processortranslating position of the surgical instrument within the virtual x-rayimaging path to a corresponding position in the true x-ray imaging path;and a display coupled to the processor for displaying a pre-acquiredx-ray image overlaid with an iconic representation of the surgicalinstrument, the position of the iconic representation of the surgicalinstrument in the pre-acquired x-ray image corresponding to thetranslated position of the surgical instrument.

Yet another system consistent with the present invention for defining asurgical plan comprises a combination of elements. The elements includean x-ray imaging device; a surgical instrument; a tracking sensor fordetecting the position, in three-dimensional space, of the surgicalinstrument; a computer processor in communication with the trackingsensor for calculating a projection of the trajectory of the surgicalinstrument a distance ahead of the actual location of the surgicalinstrument; and a display coupled to the processor for displaying apre-acquired x-ray image overlaid with an iconic representation of thesurgical instrument and the calculated projection of the trajectory ofthe surgical instrument.

Yet another system consistent with the present invention is for aligninga first bone segment with a second bone segment in a patient. The systemcomprises a first tracking marker attached to the first bone segment anda second tracking marker attached to the second bone segment. A trackingsensor detects the relative position, in three-dimensional space, of thefirst and second tracking markers. A computer delineates boundaries ofimages of the first and second bone segments in a pre-acquired x-rayimage and when the second bone segment is moved in the patient, thecomputer correspondingly moves the delineated boundary of the secondbone segment in the x-ray image. A display is coupled to the computerand displays the pre-acquired x-ray image overlaid with representationsof the delineated boundaries of the first and second bone segments.

Yet another system consistent with the present invention is directed toa system for placing a surgical implant into a patient. The systemcomprises a computer processor; means for entering dimensions of theimplant; a tracking sensor for sensing three-dimensional positioninformation of a surgical instrument on which the surgical implant isattached, the tracking sensor transmitting the position information tothe computer processor; and a memory coupled to the computer processor,the memory including computer instructions that when executed by thecomputer processor cause the processor to generate an icon representingthe surgical instrument and the attached surgical implant, and tooverlay the icon on a pre-acquired two-dimensional x-ray image, the iconof the surgical instrument representing the real-time position of thesurgical instrument relative to the pre-acquired two-dimensional x-rayimage.

In addition to the above mention devices and systems, the concepts ofthe present invention may be practiced as a number of related methods.

An additional method consistent with the present invention is a methodof acquiring a two-dimensional x-ray image of patient anatomy from adesired view direction. The method comprises generating thetwo-dimensional image using an x-ray imager; specifying a view directionin a three-dimensional image representing the patient anatomy;generating a two-dimensional digitally reconstructed radiograph (DRR)image based on the three-dimensional image and the specified viewdirection; and determining that the two-dimensional x-ray imagecorresponds to the desired view direction by matching the DRR image tothe x-ray image.

Another method consistent with the present invention is a method ofcalculating angle between a surgical instrument and a plane selected inan x-ray image. The method comprises a number of steps, including:defining at least two points in the x-ray image; defining a planepassing through the x-ray image as the plane including the two pointsand linear projections of the two points as dictated by a calibrationtransformation used to calibrate the x-ray image for its particularimaging device; sensing a position of the surgical instrument inthree-dimensional space; and calculating the angle between intersectionof a projection of the surgical instrument in three-dimensional spaceand the plane.

Yet another method consistent with the present invention is a method foraligning a fluoroscopic imager with a view direction of the medial axisof a patient's pedicle. The method comprises displaying athree-dimensional image of an axial cross-section of vertebra ofthepatient; extracting an angle from the three-dimensional imagecorresponding to the angle separating an anterior/posterior axis and themedial axis of the pedicle; aligning the fluoroscopic imager with a longaxis of the patient; and rotating the fluoroscopic imager about the longaxis of the patient through the measured angle.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments consistentwith this invention and, together with the description, help explain theprinciples of the invention. In the drawings,

FIG. 1 is a diagram of an exemplary imaging system used to acquire x-rayimages;

FIG. 2 is an image illustrating true and distorted images;

FIGS. 3A and 3B illustrate a projective transformation in a fluoroscopicC-arm imager;

FIG. 4 is a flow chart illustrating methods consistent with the presentinvention for performing two-dimensional navigational guidance;

FIGS. 5A and 5B are exemplary fluoroscopic x-ray images illustrating theiconic graphical overlay of a surgical instrument;

FIG. 6 is a fluoroscopic image including a “cross hair” graphicaloverlay of an instrument;

FIGS. 7A-7C illustrate images of complementary views and an axis thatrelates them;

FIG. 8 is an image of a lateral view of a patient's vertebral disc;

FIG. 9 is an image of a lateral view of a spinal vertebra;

FIG. 10 is a diagram illustrating a system for specifying a plannedtrajectory of a surgical instrument;

FIG. 11 is a flow chart illustrating a method for specifying a plannedtrajectory of surgical instrument;

FIGS. 12A through 12C are images of a fracture of a femur containing twobone fragments;

FIG. 13 is a flow chart illustrating methods for aligning bone fragmentsconsistent with the present invention;

FIGS. 14A and 14B are images illustrating implantation of aninter-vertebral cage in the spine of a patient;

FIGS. 15A through 15C are images used in the acquisition of an x-rayview of the medial axis of a vertebral pedicle; and

FIGS. 16A and 16B are images used to illustrate the measurement ofout-of-plane angles based on fluoroscopic images.

DETAILED DESCRIPTION

As described herein, novel methods and systems improve surgicalnavigational guidance using one or more fluoroscopic x-ray images. Themethods and systems may be used for either navigational guidance usingonly two-dimensional fluoroscopic images or for navigational guidanceusing a combination of two-dimensional fluoroscopic images andthree-dimensional volumetric images, such as CT or MRI images.

Reference will now be made in detail to embodiments of the invention,examples of which are illustrated in the accompanying drawings. Whereverpossible, the same reference number, will be used throughout thedrawings to refer to the same or like parts.

System Overview

FIG. 1 is a diagram of an exemplary imaging system used to acquire x-rayimages. Fluoroscopic imaging device 100 is a fluoroscopic C-arm x-rayimaging device that includes C-arm 103, x-ray source 104, x-rayreceiving section 105, a calibration and tracking target l06, andradiation sensors 107. Calibration and tracking target 106 includesinfrared reflectors (or alternatively infrared emitters) 109 andcalibration markers 111. C-arm control computer 115 allows a physicianto control the operation of imaging device 100, such as setting imagingparameters.

One appropriate implementation of imaging device 100 is the “Series9600Mobile Digital Imaging System,” from OEC Medical Systems, Inc., of SaltLake City, Utah, although calibration and tracking target 106 andradiation sensors 107 are typically not included in she Series9600Mobile Digital Imaging System and may have to be added. The “Series9600Mobile Digital Imaging System” is otherwise structurally similar toimaging system 100.

In operation, x-ray source 104 generates x-rays that propagate throughpatient 110 and calibration target 106, and into x-ray receiving section105. Receiving section 105 generates an image representing theintensities of the received x-rays. Typically, receiving section 105comprises an image intensifier that converts the x-rays to visible lightand a charge coupled device (CCD) video camera that converts the visiblelight to digital images. Receiving section 105 may also be a device thatconverts x-rays directly to digital images, thus potentially avoidingdistortion introduced by first converting to visible light.

Fluoroscopic images taken by imaging device 100 are transmitted tocomputer 115, where they may further be forwarded to computer 120.Computer 120 provides facilities or displaying (on monitor 121), saving,digitally manipulating, or printing a hard copy of the received images.Three-dimensional images, such as pre-acquired patient specific CT/MRdata set 124 or a three-dimensional atlas data set 126 (described inmore detail below) may also be manipulated by computer 120 and displayedby monitor 121. Images, instead of or in addition to being displayed onmonitor 121, may also be displayed to the physician through ahead-up-display.

Although computers 115 and 120 are shown as two separate computers, theyalternatively could be variously implemented as multiple computers or asa single computer that performs the functions performed by computers 115and 120. In this case, the single computer would receive input from bothC-arm imager 100 and tracking sensor 130.

Radiation sensors 107 sense the presence of radiation, which is used todetermine whether or not imaging device 100 is actively imaging. Theresult of their detection is transmitted to processing computer 120.Alternatively, a person may manually indicate when device 100 isactively imaging or this function can be built into x-ray source 104,x-ray receiving section 105, or control computer 115.

In operation, the patient is positioned between the x-ray source 104 andthe x-ray receiving section 105. In response to an operator's commandinput at control computer 115, x-rays emanate from source 104 and passthrough patient 110, calibration target 106, and into receiving section105, which generates a two-dimensional image of the patient.

C-arm 103 is capable of rotating relative to patient 110, allowingimages of patient 110 to be taken from multiple directions. For example,the physician may rotate C-arm 103 in the direction of arrows 108 orabout the long axis of the patient. Each of these directions of movementinvolves rotation about a mechanical axis of the C-arm. In this example,the long axis of the patient is aligned with the mechanical axis of theC-arm.

Raw images generated by receiving section 105 tend to suffer fromundesirable distortion caused by a number of factors, including inherentimage distortion in the image intensifier and external electromagneticfields. Drawings representing ideal and distorted images are shown inFIG. 2. Checkerboard 202 represents the ideal image of a checkerboardshaped object. The image taken by receiving section 105, however, cansuffer significant distortion, as illustrated by distorted image 204.

The image formation process in a system such as fluoroscopic C-armimager 100 is governed by a geometric projective transformation whichmaps lines in the fluoroscope's field of view to points in the image(i.e., within the x-ray receiving section 105). This concept isillustrated in FIGS. 3A and 3B. Image 300 (and any image generated bythe fluoroscope) is composed of discrete picture elements (pixels), anexample of which is labeled as 302. Every pixel within image 300 has acorresponding three-dimensional line in the fluoroscope's field of view.For example, the line corresponding to pixel 302 is labeled as 304. Thecomplete mapping between image pixels and corresponding lines governsprojection of objects within the field of view into the image. Theintensity value at pixel 302 is determined by the densities of theobject elements (i.e., portions of a patient's anatomy, operating roomtable, etc.) intersected by the line 304. For the purposes of computerassisted navigational guidance, it is necessary to estimate theprojective transformation which maps lines in the field of view topixels in the image, and vice versa. Geometric projective transformationis well known in the art.

Intrinsic calibration, which is the process of correcting imagedistortion in a received image and establishing the projectivetransformation for that image, involves placing “calibration markers” inthe path of the x-ray, where a calibration marker is an object opaque orsemi-opaque to x-rays. Calibration markers 111 are rigidly arranged inpredetermined patterns in one or more planes in the path of the x-raysand are visible in the recorded images. Tracking targets, such asemitters or reflectors 109, are fixed in a rigid and known positionrelative to calibration markers 111.

Because the true relative position of the calibration markers 111 in therecorded images are known, computer 120 is able to calculate an amountof distortion at each pixel in the image (where a pixel is a singlepoint in the image). Accordingly, computer 120 can digitally compensatefor the distortion in the image and generate a distortion-free, or atleast a distortion improved image. Alternatively, distortion may be leftin the image, and subsequent operations on the image, such assuperimposing an iconic representation of a surgical instrument on image(described in more detail below), may be distorted to match the imagedistortion determined by the calibration markers. The same calibrationmarkers can also be used to estimate the geometric perspectivetransformation, since the position of these markers are known withrespect to the tracking target emitters or reflectors 109 and ultimatelywith respect to tracking sensor 130. A more detailed explanation ofmethods for performing intrinsic calibration is described in thereferences B. Schuele et al., “Correction of Image IntensifierDistortion for Three-Dimensional Reconstruction,” presented at SPIEMedical Imaging 1995, San Diego, Calif., 1995 and G. Champleboux et al.,“Accurate Calibration of Cameras and Range Imaging Sensors: the NPBSMethod,” Proceedings of the 1992 IEEE International Conference onRobotics and Automation, Nice, France, May 1992, and U.S. applicationSer. No. 09/106,109, filed on Jun. 29, 1998 by the present assignee, thecontents of which are hereby incorporated by reference.

Calibration and tracking target 106 may be attached to x-ray receivingsection 105 Of the C-arm. Alternately, the target 106 can bemechanically independent of the C-arm, in which case it should bepositioned such that the included calibration markers 111 are visible ineach fluoroscopic image to be used in navigational guidance. Element 106serves two functions. The first, as described above, is holdingcalibration markers 111 used in intrinsic calibration. The secondfunction, which is described in more detail below, is holding infraredemitters or reflectors 109, which act as a tracking target for trackingsensor 130.

Tracking sensor 130 is a real-time infrared tracking sensor linked tocomputer 120. Specially constructed surgical instruments and othermarkers in the field of tracking sensor 130 can be detected and locatedin three-dimensional space. For example, a surgical instrument 140, suchas a drill, is embedded with infrared emitters or reflectors 141 on itshandle. Tracking sensor 130 detects the presence and location ofinfrared emitters or reflectors 141. Because the relative spatiallocations of the emitters or reflectors in instrument 140 are known apriori, tracking sensor 130 and computer 120 are able to locateinstrument 140 in three-dimensional space using well known mathematicaltransformations. Instead of using infrared tracking sensor 130 andcorresponding infrared emitters or reflectors, other types of positionallocation devices are known in the art, and may be used. For example, apositional location device may also be based on magnetic fields, sonicemissions, or radio waves.

Reference frame marker 150, like surgical instrument 140, is embeddedwith infrared emitters or reflectors, labeled 151. As with instrument140, tracking sensor 130 similarly detects the spatial location ofemitters/reflectors 151, through which tracking sensor 130 and computer120 determine the three-dimensional position of dynamic reference framemarker 150. The determination of the three-dimensional position of anobject relative to a patient is known in the art, and is discussed, forexample, in the following references, each of which is herebyincorporated by reference: PCT Publication WO 96/11624 to Bucholz etal., published Apr. 25, 1996; U.S. Pat. No. 5,384,454 to Bucholz; U.S.Pat. No. 5,851,183 to Bucholz; and U.S. Pat. No. 5,871,445 to Bucholz.

During an operation, dynamic reference frame marker 150 is attached in afixed position relative to the portion of the patient to be operated on.For example, when inserting a screw into the spine of patient 110,dynamic reference frame marker 150 may be physically attached to aportion of the spine of the patient. Because dynamic reference frame 150is in a fixed position relative to the patient anatomy, and instrument140 can be accurately located in three dimensional space relative todynamic reference frame 150, instrument 140 can also be located relativeto the patient's anatomy.

As discussed above, calibration and tracking target 106 also includesinfrared emitters or reflectors 109 similar to those in instrument 140or dynamic reference frame 150. Accordingly, tracking sensor 130 andcomputer 120 may determine the three-dimensional position of calibrationtarget 106 relative to instrument 140 and/or dynamic reference frame 150and thus the patient position.

System Operation

In general, the imaging system shown in FIG. 1 assists physiciansperforming surgery by displaying real-time or pre-acquired images, suchas fluoroscopic x-ray images, of patient 110 on display 121.Representations of surgical instruments 140 are overlaid on pre-acquiredfluoroscopic images of patient 110 based on the position of theinstruments determined by tracking sensor 130. In this manner, thephysician is able to see the location of the instrument relative to thepatient's anatomy, without the need to acquire real-time fluoroscopicimages, thus greatly reducing radiation exposure to the patient and tothe surgical team. “Pre-acquired,” as used herein, is not intended toimply any required minimum duration between receipt of the x-ray signalsand displaying the corresponding image. Momentarily storing thecorresponding digital signal in computer memory while displaying thefluoroscopic image constitutes pre-acquiring the image.

FIG. 4 is a flow chart illustrating methods consistent with the presentinvention for performing two-dimensional navigational guidance using thesystem of FIG. 1. The physician begins by acquiring one or morefluoroscopic x-ray images of patient 110 using imager 100 (step 400). Aspreviously mentioned, acquiring an x-ray image triggers radiationsensors 107, which informs computer 120 of the beginning and end of theradiation cycle used to generate the image. For a fluoroscopic x-rayimage acquired with imager 100 to be useable for navigational guidance,imager 100, when acquiring the image, should be stationary with respectto patient 110. If C-arm 103 or patient 110 is moving during imageacquisition, the position of the fluoroscope will not be accuratelydetermined relative to the patient's reference frame. Thus, it isimportant that the recorded position of imager 100 reflects the trueposition of the imager at the time of image acquisition. If imager 100moves during the image acquisition process, or if imager 100 moves afterimage acquisition but before its position is recorded the calibrationwill be erroneous, thus resulting in incorrect graphical overlays. Toprevent this type of erroneous image, computer 120 may examine theposition information from tracking sensor 130 while radiation sensors107 are signaling radiation detection. If the calibration and trackingtarget 106 moves relative to dynamic reference frame 150 during imageacquisition, this image is marked as erroneous. (Steps 401 and 402).

At the end of the radiation cycle, computer 120 retrieves the acquiredimage from C-arm control computer 115 and retrieves the locationinformation of target marker 106 and dynamic reference frame 150 fromtracking sensor 130. Computer 120 calibrates the acquired image, asdescribed above, to learn its projective transformation and optionallyto correct distortion in the image, (step 1403), and then stores theimage along with its positional information (step 404). The process ofsteps 400-404 is repeated for each image that is to be acquired (step405).

Because the acquired images are stored with the positional informationof the calibration and tracking target 106 and dynamic reference frame150, the position of C-arm 103, x-ray source 104, and receiving section105 for each image, relative to patient 110, can be computed based uponthe projective transformation identified in the calibration process.During surgery, tracking sensor 130 and computer 120 detect the positionof instrument 140 relative to dynamic reference frame 150, and hencerelative to patient 110. With this information, computer 120 dynamicallycalculates, in real-time, the projection of instrument 140 into eachfluoroscopic image as the instrument is moved by the physician. Agraphical representation of instrument 140 may then be overlaid on thefluoroscopic images (step 406). The graphical representation ofinstrument 140 is an iconic representation of where the actual surgicalinstrument would appear within the acquired fluoroscopic x-ray image ifimager 100 was continuously acquiring new images from the same view asthe original image. There is no theoretical limit to the number offluoroscopic images on which the graphical representations of instrument140 may be simultaneously overlaid.

FIGS. 5A and 5B are exemplary fluoroscopic x-ray images illustrating theiconic graphical overlay of a surgical instrument. Fluoroscopic image500, shown in FIG. 5A, is an image of a lateral view of the lumbarspine. Graphical overlay 502 is the iconic overlay of a surgicalinstrument, such as a drill, within image 500. As the physician movesthe drill, computer 120 recalculates and displays the new location ofgraphical overlay 502. The diamond shaped end of overlay 502, labeled asarea 503, represents the tip of the instrument. The physician can useimage 500 and overlay 502 to visualize the position and orientation ofthe surgical tool relative to the patient's anatomy.

In certain situations, the physician may wish to know where the tip ofthe instrument would be if the instrument were projected along a linegiven by the instrument's current trajectory. Consistent with an aspectof the present invention, at the physician's command, computer 120 maycalculate and display this projection. Area 505 in FIG. 5B is aprojection of graphical overlay 502. As shown, the “look-ahead”trajectory 505 of overlay 502 is displayed in a different line stylethan overlay 502. Computer 120 may vary the length of look-aheadtrajectory 505 as directed by the physician through a suitable computerinterface device, such as a keypad, mouse, or touch pad. In this manner,computer 120 assists the physician in visualizing where the instrumentwould be in the patient if it were advanced a predetermined distance inthe patient.

Although the “look-ahead” technique described above projected thegraphical representation of the instrument into the image, there is norequirement that the instrument's graphical representation be in thespace of the image for look-ahead trajectory 505 to be projected intothe image. For example, the physician may be holding the instrumentabove the patient and outside the space of the image, so that therepresentation of the instrument does not appear in the image. However,it may still be desirable to project look-ahead portion 505 into theimage to facilitate planning of a surgical procedure.

When surgical instrument 140 is perpendicular to the plane of thefluoroscopic image, the graphical overlay of the surgical instrumentessentially collapses to a point, making it difficult to view. Toalleviate this problem, computer 120 may optionally use a differentgraphical representation of instrument 140 when the distance in theimage plane between the tip and the tail of instrument 140 becomessmaller than a fixed distance (e.g., 15 pixels).

FIG. 6 is a fluoroscopic image including graphical overlay 601 ofinstrument 140, including a small “cross hair image” representing tip602 and a larger cross hair representing tail 603 of instrument 601.Computer 120 automatically switches between the cross hairrepresentation shown in FIG. 6 and the “straight line” representationshown in FIG. 5.

Frequently, the physician would like to acquire two complementaryfluoroscopic images of the patient, such as images from ananterior/posterior view and a lateral view of the vetebral discs. Thecomplementary views are related to one another by a rotation about anaxis by a particular amount. For example, an anterior/posterior view isrelated to a lateral view by a 90 degree rotation around the axisrunning parallel through the length of the patient. When the mechanicalaxis of rotation of C-arm 103 is aligned with the axis relating thecomplementary views (e.g., when the mechanical axis is aligned with theaxis running through the length of the patient), the physician canaccurately and quickly switch between the complementary views by simplyrotating C-arm 103 through the separation of the complementary views(usually 90 degrees). Generally, however, the axis of rotation of C-arm103 is not inherently aligned with the axis that relates thecomplementary views, requiring the physician to perform a series of timeconsuming trial-and-error based adjustments of the fluoroscope'sposition through two or more axes of rotation.

Consistent with an aspect of the present invention, software on computer120 allows the surgeon to easily adjust the fluoroscope's position sothat one of its mechanical rotation axes, such as the axis of rotationshown by arrows 108 in FIG. 1, is aligned with the axis of rotationrelating the complementary views. The surgeon may then acquire thesecond image in the complementary image set simply by rotating C-arm 103a certain amount, typically 90 degrees, about the aligned axis.

Images of complementary views and the axis that relates them areillustrated in FIGS. 7A-7C. The image of FIG. 7A illustrates a lateralview of the patient's vertebral disc, in which the view direction (i.e.,the direction of the central ray of fluoroscopic imager 100) isapproximately parallel to the two vertebral end plates, labeled asendplate 705 and endplate 706. Line 702 is the projection of the planesubstantially parallel to end plates 705 and 706. Similarly, the imageshown in FIG. 7B is an anterior/posterior view of the patient'svertebral disc, in which the view direction is parallel to plane 702.The axis of rotation 704 that spatially relates the image view of FIG.7A and the image view of FIG. 7B is a line perpendicular to plane 702.That is, rotating the image view of FIG. 7A ninety degrees about theline perpendicular to plane 702 will result in the image view shown inFIG. 7B. FIG. 7C is a three-dimensional representation of the anatomyshown in FIGS. 7A and 7B. The line perpendicular to plane 702 is shownby axis of rotation 704.

FIG. 8 is an image of a lateral view of the patient's vertebral disc,similar to FIG. 7A. In FIG. 8, however, computer 120 has drawn line 802,which represents the projection of a plane that is perpendicular to oneof the C-arm's mechanical axes. Line 804 represents the plant, thatspatially relates the complementary views. With line 802 visible, thephysician may adjust the position of fluoroscopic imager 100 so thatline 802 is lined up with line 804. At this point, switching between thecomplementary views simply involves rotating C-arm 103 about a singlemechanical axis.

Although the alignment of lines 802 and 804, as discussed above, wasillustrated using both lines 802 and 804 drawn on the fluoroscopicimage, in practice, it may only be necessary to display line 802 in theimage. In this case, line 804 is mentally visualized by the physician.Additionally, although the relation of complimentary views was discussedusing the example of the spine, complimentary fluoroscopic images ofother anatomical regions, such as, for example, the pelvis, femur, orcranium, may similarly be obtained by application of the above discussedconcepts.

Before, or during, surgery, the physician may find it desirable to inputan operation “plan” to computer 120. The plan may, for example, specifya desired trajectory of a surgical instrument superimposed on afluoroscopic image. During the surgical navigation process, the goal ofthe surgeon would be to align the graphical icon representing thereal-time location of the surgical instrument with the graphical overlayrepresenting the planned trajectory.

FIG. 9 is an image of a lateral view of a spinal vertebra. Assume thegoal of the operation plan is to define a line that passes along adesired trajectory within the image of the vertebra. One method ofaccomplishing this goal is to directly input the desired trajectoryinformation to computer 120 using traditional computer input devices.While this method of directly interacting with computer 120 is possible,it can be cumbersome and disruptive during surgery. Consistent with anaspect of the present invention, an alternative method of accomplishingthis is for the physician to position the surgical instrument on thesurface of the bone or skin in the desired orientation, and then projectthe tip of the instrument forward using the previously describedlook-ahead technique. More specifically, the desired trajectory isspecified by (1) adjusting the position and orientation of theinstrument near the patient with virtual look-ahead active, and (2)adjusting the length of the virtual look-ahead. FIG. 9 illustrates theiconic representation of instrument 901 and the virtual look-aheadprojection of the instrument 902. Once the desired trajectory isachieved, the surgeon may direct computer 120 to “freeze” the plannedtrajectory on display 121. The desired trajectory can be obtained usingone or more C-arm fluoroscopic images with two or more being required todefine a specific three-dimensional trajectory which can then bedisplayed on any C-arm fluoroscopic view. The freeze operation may beinput to computer 120 through, for example, a simple input device suchas a foot pedal. The physician may then proceed with the operation,using the overlay of the planned target as a guide.

Yet another method consistent with the present invention for specifyinga planned trajectory of a surgical instrument, which, unlike the methoddiscussed above, does not require positioning the surgical instrument onor near the patient's bone, is illustrated in FIGS. 10 and

As shown in FIG. 10, during the acquisition of an image, patient 1001 ispositioned between C-arm x-ray source 1004 and x-ray receiving section1006. Fluoroscopic images of patient 1001 are created by the x-raysemitted from x-ray source 1004 as they travel in the path generallyoutlined by cone 1010 through patient 1001. Line 1011, in the center ofcone 1010, corresponds to the origin (i.e., the center point) in theacquired fluoroscopic images. Physician 1020, standing within the rangeof tracking sensor 1030, but away from patient 1001, commands thecomputer to create an explicit correspondence between the fluoroscope'simaging cone 1010 and a “virtual” cone 1012 at an arbitrary position inspace (which is visible to the tracking sensor). Once this virtual conehas been defined, the surgical instrument 1040 can be projected fromthis virtual cone into one or more pre-acquired fluoroscopic images thesame manner as if the instrument were located in the actual cone 1010corresponding to a given image. In this manner, physician 1020 can planthe trajectory of surgical instrument 1040 by simply moving theinstrument in the coordinate system established by the virtual cone.

To define the correspondence between actual and virtual cones, it isnecessary for the physician to define the position of the virtual conerelative to the tracking sensor. In general, there are many ways todefine a cone in space. For example, the position and orientation of acone can be defined by three points, one corresponding to its apex, onecorresponding to a second point along its central axis, and a thirdcorresponding to the rotation of the cone about the central axis.Therefore, one way to define the cone would be to use the tip of thesurgical instrument to define these three points in space relative tothe tracking sensor. Another way to define this correspondence is to usea single measurement of a surgical instrument. Using this method, theaxis of the instrument corresponds to the axis of the cone, the tip ofthe instrument corresponds to a fixed point along the axis of the cone(which could be the apex, but could also be another point along thecentral axis), and the orientation of the instrument about its axiscorresponds to the orientation of the cone about its axis. In generalany set of measurements which define the position and orientation of agiven cone can be used to establish the correspondence between theactual and virtual cones.

The operations illustrated in FIG. 10 are shown in the flowchart of FIG.11. To begin, the physician holds the surgical instrument 1040 in theposition that defines the virtual cone in the manner as outlined in theprevious paragraph (step 1101). Computer 120 locates the position ofinstrument 1040, which effectively corresponds the position andorientation of the virtual cone to the actual cone (step 1102). Computer120 projects additional movements of instrument 1040 into one or morepreviously acquired fluoroscopic images as if the instrument were beingmoved in the actual cone corresponding to a given image (step 1103). Inthis manner, the physician can align the instrument to particular pointsor trajectories within previously acquired images. At the physician'scommand, computer 120 “freezes” the position and/or orientation of theinstrument in the displayed fluoroscopic image(s) and uses those forsubsequent processing and plan generation (step 1104).

It is also consistent with this invention to provide automated planningusing computer analysis techniques to define an “optimal” trajectory inthe C-arm images. Once the optimal trajectory is determined, computer120 overlays the optimal trajectory in the fluoroscopic image. Forexample, automated plans can be generated using computational techniquesto reduce a specified amount of lordosis in spine surgery.

Alignment of Bone Fragments

A common clinical problem, especially in orthopaedic trauma, is therealignment (reduction) of broken or misaligned bone fragments. FIG. 12Ais a fluoroscopic image of a fracture of the femur containing two bonefragments 1201 and 1202. The physician's job is to realign the bonefragments so that the femur can properly heal.

FIG. 13 is a flow chart illustrating methods for aligning bone fragmentsconsistent with the present invention. In general, one of bone fragments1201 or 1202 is used as a fixed reference frame and the other as adynamic reference frame. When the physician moves the bone fragmentcorresponding to the dynamic reference frame, tracking sensor 130detects the movement and updates the x-ray image to reflect the newlocation of the bone fragment it the patient.

To begin the alignment procedure, the physician places a tracking sensormarker on each of bone fragments 1201 and 1202 (step 1301) and acquiresthe fluoroscopic images, (step 1302), such as the image shown in FIG.12A. Computer 120 processes the acquired image to obtain positionallocation information and to calibrate the image (step 1303, this step isidentical to steps 401-403 in FIG. 4).

After acquisition of the fluoroscopic image(s), computer 120 uses imagedetection and extraction techniques to delineate the boundaries of thebone fragments in the images (step 1304). Suitable edge detectionalgorithms for generating the contours are well known in the art, andmay be, for example, the Canny edge detector, the Shen-Casten edgedetector, or the Sobel edge detector. An edge detected version of FIG.12A is shown in FIG. 12B, in which the resulting contour correspondingto bone fragment 1201 is labeled as 1203 and the contour correspondingto bone fragment 1202 is labeled as 1204. Contours 1203 and 1204 may be,as shown in FIG. 12B, graphically superimposed by computer 120 on theacquired image(s).

Overlaying the detected image contours on the fluoroscopic image allowsthe physcian to easily identify the correspondence between imagecontours 1203-1204 and bone fragments 1201-1202. The physician inputsthis correspondence into computer 120 (step 1305). Alternatively,computer 120 may automatically identify the correspondence between theimage contours and the bone fragments. Once the correspondence isestablished, the physician specifies which contour is to remain fixedand which is to be repositioned. The tracking sensor marker attached tothe fragment to be repositioned is referred to as the dynamic referencemarker and the tracking sensor marker attached to the fixed fragment isreferred to as the fixed reference frame marker, although physically thedynamic reference marker and the fixed reference frame marker may beidentical.

During surgical navigation, the physician moves the bone fragment havingthe dynamic reference marker (step 1306). Tracking sensor 130 detectsthe position of the dynamic reference frame marker and the fixed framemarker. With this information and the previously generated positionallocation information, computer 120 calculates and displays the newposition of the dynamic reference frame, and hence its correspondingbone fragment, in the fluoroscopic image (step 1307). FIG. 12Cillustrates an updated version of the fluoroscopic image contour 1203corresponding to the fixed bone fragment and contour 1204 correspondingto the new location of the dynamic reference marker and its bonefragment.

Methods described above for aligning bone fragments may also be appliedto the proper alignment of multiple vertebral bodies, for example in thereduction of scoliosis.

Three-Dimensional Images

The navigational guidance system consistent with the present inventionis not limited to providing surgical navigational guidance withtwo-dimensional fluoroscopic images. Three-dimensional volumetric datasets may also be overlaid with graphical representations of a surgicalinstrument. Three-dimensional data sets (such as CT or MRI) may beeither pre-acquired or acquired during the operation.

Two types of three-dimensional data sets are typically used in surgicalnavigation: patient-specific image data and non-patient specific oratlas data. Patient-specific three-dimensional images are typicallyacquired prior to surgery using computed tomography (CT), magneticresonance (MR), or other known three-dimensional imaging modalities,although intra-operative acquisition is also possible. Atlas data isnon-patient specific three-dimensional data describing a “generic”patient. Atlas data may be acquired using CT, MR or other imagingmodalities from a particular patient; and may even comprise images fromseveral modalities which are spatially registered (e.g., CT and MRtogether in a common coordinate system). Atlas data may be annotatedwith supplemental information describing anatomy, physiology, pathology,or “optimal” planning information (for example screw placements,lordosis angles, scoliotic correction plans, etc).

A three-dimensional patient CT or MR data set is shown in FIG. 1 as dataset 124 and atlas data is illustrated in FIG. 1 as data set 126.

Before overlaying a three-dimensional image with graphicalrepresentations of surgical instruments, the correspondence betweenpoints in the three-dimensional image and point in the patient'sreference frame must be determined. This procedure is known asregistration of the image. One method for performing image registrationis described in the previously mentioned publications to Bucholz.Three-dimensional patient specific images can be registered to a patienton the operating room table (surgical space) using multipletwo-dimensional image projections. This process, which is often referredto as 2D/3D registration, uses two spatial transformations that can beestablished. The first transformation is between the acquiredfluoroscopic images and the three-dimensional image data set (e.g., CTor MR) corresponding to the same patient. The second transformation isbetween the coordinate system of the fluoroscopic images and anexternally measurable reference system attached to the fluoroscopicimager. Once these transformations have been established, it is possibleto directly relate surgical space to three-dimensional image space.

When performing three-dimensional registration, as with two-dimensionalregistration, imager 100, when acquiring the image, should be stationarywith respect to patient 110. If C-arm 103 or patient 110 is movingduring image acquisition, the position of the fluoroscope will not beaccurately determined relative to the patient's reference frame.Accordingly, the previously described technique for detecting movementof imager 100 during the image acquisition process can be used whenacquiring fluoroscopic images that are to be used in 2D/3D registration.That is, as described, computer 120 may examine the position informationfrom tracking sensor 130 while radiation sensors 107 are signalingradiation detection. If the calibration and tracking target 106 movesrelative to dynamic reference frame 150 during image acquisition, thisimage is marked as erroneous.

It may be necessary to acquire complementary fluoroscopic views (e.g.,lateral and anterior/posterior) to facilitate 2D/3D registration. Thetechniques previously discussed in reference to FIGS. 7-8 and relatingto the acquisition of complementary views can be applied here.

Once registered, computer 120 may use positional information ofinstrument 140 to overlay graphical representations of the instrument inthe three-dimensional image as well as the two-dimensional fluoroscopicimages.

Hybrid Use of Three-Dimensional and Two-Dimensional Image Data

The two-dimensional images generated by imager 100 are not always ableto adequately represent the patient's bone structure. For example,fluoroscopic x-ray images are not effective when taken through thelength of the patient (i.e., from the point of view looking down at thepatient's head or up from the patient's feet) because the large numberof bones that the x-rays pass through occlude one another in the finalimage. However, information required for planning a surgical procedurewhich is not otherwise available based on two-dimensional image dataalone may be extracted from a three-dimensional image data set such as aCT or MR image data set. The extracted information may then betransferred to the two-dimensional x-ray images generated by imager 100and used in surgical navigation. The following examples describeadditional methods for using three-dimensional and two-dimensional datain surgical navigation.

EXAMPLE 1

Importing Three-Dimensional Surgical Implant Specifications toTwo-Dimensional Images

FIGS. 14A and 14B are images illustrating the implantation of aninter-vertebral cage in the spine of a patient. An inter-vertebral cageis a roughly cylindrical spinal implant that is inserted in the discspace between adjacent spinal vertebrae. The physician may find itdifficult, if not impossible, to choose the appropriate length of aninter-vertebral cage based upon two-dimensional images such as the imageof FIG. 14A.

Rectangle 1401 represents the projection of the cylindricalinter-vertebral cage into the image. While the long axis of the cylinderappears to be completely within the bone in this image, this may not bethe case due to curvature of the anterior aspect of vertebrae 1402. FIG.14B is an image of a three-dimensional axial CT cross section of thevertebrae. Corner 1403 of rectangle 1401 protrudes from the bone-ahighly undesirable situation that cannot be reliably detected in x-rayimages such as that of FIG. 14A. Accordingly, when faced with thissituation, the appropriate cage length should be chosen based upon oneor more axial CT images, such as that in FIG. 14B. Selection of the cagelength can be performed automatically by computer 120 orsemi-automatically with the input of the physician.

Once the cage length has been determined by the physician and enteredinto computer 120, the length value can then be used by computer 120 inproperly displaying the graphical overlay in the associatedtwo-dimensional image. The position of the surgical instrument used tohold the cage during the insertion process, as detected by trackingsensor 130, is used to calculate the position of the cage in FIG. 14Aduring the two-dimensional navigational process.

Although the above discussed example was with a cylindrical spinalimplant, in general, the described concepts could be applied to anysurgical implant.

EXAMPLE 2

Acquisition of an X-Ray View Down the Medial Axis of a Vertebral Pedicle

In certain clinical procedures, it may be desirable to acquire afluoroscopic x-ray image view looking substantially straight down themedial axis of a vertebral pedicle. For the purposes of this example, avertebral pedicle can be thought of as a cylinder, and the medial axiscorresponds to the central axis of the cylinder.

FIG. 15A is an x-ray image in which the view direction of the imager isaligned with the medial axis of the pedicle (i.e., the medial axis ofthe pedicle is into the plane of the image. In this so-called “owl'seye” view, the pedicle appears as circle 1501 within the image. It isoften difficult to precisely acquire this view using only fluoroscopicx-ray images, as it is difficult it to align the view direction ofimager 100 with the medial axis of the pedicle using only fluoroscopicimages.

Given an anterior/posterior fluoroscopic image view of the spine, suchas the one shown in FIG. 15B, and given that the mechanical axis of thefluoroscope is aligned with the patient's long axis (i.e., axis 704 inFIG. 7C), an axial CT cross section of a vertebra can be used to quicklyand easily acquire a high quality owl's eye view, such as the view ofFIG. 15A.

FIG. 15C is an image of an axial CT cross section of a vertebra. Withthis image, computer 120 or the physician may measure angle 1504 betweenthe anterior/posterior axis 1502 and the projection of the medial axis1503 of the pedicle 1501 into the axial plane. The physician may thenrotate imager 100 by the measured angle about the mechanical rotationaxis that is aligned with the patient's long axis 704. Because mostfluoroscopic imagers such, as imager 100, have angle indicators,rotation by the desired amount is trivial. However, if the physicianrequires additional accuracy in the rotation, tracking sensor 130,because it detects the position of C-arm 103, can be used to moreprecisely measure the rotation angle.

EXAMPLE 3

Use of Digitally Reconstructed Radiography in the Placement of aSurgical Implant

With conventional fluoroscopic x-ray image acquisition, radiation passesthrough a physical media to create a projection image on a radiationsensitive film or an electronic image intensifier. Given a 3D CT dataset, a simulated x-ray image can also be generated using a techniqueknown as digitally reconstructed radiography (DRR). DRR is well known inthe art, and is described, for example, by L. Lemieux et al., “APatient-to-Computed-Tomography Image Registration Method Based onDigitally Reconstructed Radiographs,” Medical Physics 21(11), pp1749-1760, November 1994.

When a DRR image is created, a fluoroscopic image is formed bycomputationally projecting volume elements (voxels) of the 3D CT dataset onto a selected image plane. Using a 3D CT data set of a givenpatient, it is possible to create a DRR image that appears very similarto a corresponding x-ray image of the same patient. A requirement forthis similarity is that the “computational x-ray imager” and actualx-ray imager use similar intrinsic imaging parameters (e.g., projectiontransformations, distortion correction) and extrinsic imaging parameters(e.g., view direction). The intrinsic imaging parameters can be derivedfrom the calibration process.

A DRR image may be used to provide guidance to the surgeon in theproblem discussed in Example 1 of appropriately placing aninter-vertebral cage in the patient. Given a 3D CT data set of twoadjacent vertebrae, the physician, interacting with computer 120, maymanually position a 3D CAD model of an inter-vertebral cage in aclinically desired position in the three-dimensional view of thevertebrae. The physician may then use the DRR technique to synthesize ananterior/posterior, lateral, or other x-ray view of the vertebraeshowing the three-dimensional CAD model of the inter-vertebral cage.Thus, a synthetic fluoroscopic x-ray image can be created whichsimulates what a properly placed cage would look like afterimplantation.

The simulated x-ray images may be compared to the actual images taken byimager 100 during surgery. The goal of the surgeon is to position theimplant such that the intra-operative images match the DRR images. Forthis comparison, two types of intra-operative images may preferably beused. First, conventional fluoroscope could be used to acquire an imageafter the inter-vertebral cage has been implanted. Second, imagesacquired prior to cage placement could be supplemented with superimposedgraphical icons representing the measured cage position. In either case,the synthetic fluoroscopic image can be used as a template to help guidethe surgeon in properly placing the inter-vertebral cage.

Although the above example was described in the context of implanting aninter-vertebral cage, implants other than the inter-vertebral cage couldalso be used.

EXAMPLE 4

Obtaining a Particular Two-Dimensional View Direction Using DigitallyReconstructed Radiograph Images

The DRR technique can be used to provide guidance to the physician whenacquiring an owl's eye view of a vertebral pedicle. Given athree-dimensional CT data set containing a vertebra and associatedpedicle, the physician may use computer 120 to manually locate athree-dimensional representation of the pedicle's medial axis relativeto the three-dimensional images of the vertebrae. Once this placementhas been achieved, it is possible to synthesize an owl's eye view of thevertebrae based upon the view direction specified by the physician'sselection of the three-dimensional medial axis. This synthetic image canthen be displayed to the surgeon during surgery and used to guide theacquisition of an actual owl's eye view using the fluoroscope. Byvisually comparing fluoroscopic images taken while positioning thefluoroscope to the synthetic owl's eye view, the physician can acquire atrue fluoroscopic image with a view direction approximately equal to themanually selected medial axis. In this manner, a high quality owl's eyeview can be acquired.

Although the above example was described in the context of synthesizinga two-dimensional owl's eye view, in general, any three-dimensional viewdirection can be selected and a corresponding two-dimensional imagesynthesized and used to acquire a fluoroscopic two-dimensional image.

EXAMPLE 5

Measuring Out-Of-Plane Angles Based on Fluoroscopic Images

It may be desirable to measure the angle between the trajectory of asurgical instrument and the plane of a fluoroscopic image (such as aplane aligned with the mid-line of the spine 1502) during surgery usinga pre-acquired fluoroscopic image. This is useful, as it is oftendesirable to position or implant a surgical instrument at a certainangle relative to the plane of the fluoroscopic image. For example, thesurgical instrument may need to be implanted in the direction alignedwith the medial axis of the pedicle 1503.

Consider the vertebral cross section shown as an axial CT image in FIG.15C. As described above, the angle 1504 between the anterior/posterioraxis of the spine 1502 and the medial axis 1503 of the pedicle can bemeasured from this CT image. Aligning the surgical instrument with themedial axis can be accomplished by dynamically measuring the anglebetween the trajectory of the surgical instrument and the plane definedby the mid-line of the spine 1502. When the dynamically measured anglematches the angle pre-obtained from the CT image, the surgicalinstrument is aligned.

FIGS. 16A and 16B are figures respectively illustrating ananterior/posterior fluoroscopic image of the spine and a correspondingthree-dimensional view of the spine. The physician defines two pointsalong the midline of the spine, such as the points 1601 drawn on thespinous processes in FIG. 16A (in non-pathological anatomy a spinousprocess typically defines the midline). Computer 120 uses these pointsto define a line 1602 in the image, or more generally, the computerdefines plane 1603 (shown in FIG. 16B) to include the two points and thelinear projections of these two points dictated by the calibrationtransformation. More intuitively, a first order approximation of plane1603 can be thought of as the plane passing through the two pointsperpendicular to the image plane. Plane 1603 defines the midline of thespine in three-dimensional space. During navigational guidance, theequation of this plane can be expressed in the coordinate system ofeither the dynamic reference frame 150 or the tracking sensor 130.

Using the tracking sensor 130 to measure the position and orientation(i.e., the trajectory) of the instrument 140, computer 120 thenmathematically projects this trajectory onto the plane 1603. Thisprojection will define a line passing through plane 1603. The anglebetween this line in plane 1603 and the instrument trajectorycorresponds to the angle to be measured. In other words, the angle to bemeasured corresponds to the minimum angle present between the trajectoryof the instrument and the plane 1603. The angle to be measured can becalculated by computer 120 and displayed to the physician either in atextual or graphical format.

In summary, as described in this example, a single fluoroscopic imagecan be used during surgery to position a surgical instrument at adesired trajectory relative to the plane of the fluoroscopic image. Moregenerally, the methods described in this example relate to measuring theangle between the trajectory of a surgical instrument 140 and a plane(e.g. 1603) defined by two or more points (e.g., 1601) which have beenmanually or automatically selected in a fluoroscopic image. While theexplanation uses a CT for clarity of the example, the measurement anddisplay of the angle can be achieved without the use of any 3D imagedata.

Although the above five examples used three-dimensional patient specificdata and not atlas data, in certain situations, it may be possible touse a 2D/3D registration scheme that registers non-patient specificatlas data to patient specific fluoroscopic images using deformableregistration methods that do not preserve the rigidity of anatomicalstructure during the registration process. In this manner, the patientspecific fluoroscopic images may be used to deform the atlas data tobetter correspond to the patient and thereby transfer atlased knowledgeto the patient specific fluoroscopic images.

Conclusion

The above described systems and methods significantly extend theconventional techniques for acquiring and using x-ray images forsurgical navigational guidance. It will be apparent to those skilled inthe art that various modifications and variations can be made to thepresent invention without departing from the scope or spirit of theinvention. For example although certain of the examples were describedin relation to spinal examples, many other regions of body could beoperated on.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. In particular, an alternative embodiment ofthe calibration and tracking target may allow the calibration componentto be detached from the C-arm and introduced into the C-arm view forcalibrations only, and then removed. It is intended that thespecification and examples be considered as exemplary only, with thetrue scope and spirit of the invention being indicated by the followingclaims.

What is claimed:
 1. An x-ray imaging device comprising: an x-ray sourcefor generating cycles of x-ray radiation corresponding to an imageacquisition cycle; an x-ray receiving section positioned so that x-raysemanating from the x-ray source enter the x-ray receiving section, thex-ray receiving section generating an image representing intensities ofthe x-rays entering the x-ray receiving section; a computer coupled tothe x-ray receiving section; radiation sensors located in a path ofx-rays emitted from the x-ray source, the radiation sensors detecting abeginning and an end of a radiation cycle and transmitting the detectedbeginning and end of the radiation cycle to the computer; a dynamicreference frame marker attached in a fixed position relative to apatient; a calibration and tracking target; and a tracking sensor fordetecting a position, in three-dimensional space, of the dynamicreference frame marker and the calibration and tracking target; whereinthe computer determines that an image acquisition cycle is erroneouswhen the, position of the calibration and tracking target moves relativeto the position of the dynamic reference frame during an imageacquisition cycle.
 2. The device of claim 1, wherein the computerfurther comprises a display for displaying the image generated by thex-ray receiving section.
 3. The device of claim 1, wherein thecalibration and tracking target includes calibration markers and thecalibration and tracking target is attached to the imaging device. 4.The device of claim 1, wherein the calibration and tracking target isphysically separated from the x-ray source and the x-ray receivingsection and calibration markers included in the calibration and trackingtarget are in the path of x-rays of the generated cycles of x-rayradiation.
 5. An x-ray imaging device comprising: an x-ray source forgenerating cycles of x-ray radiation corresponding to an imageacquisition cycle; an x-ray receiving section positioned so that x-raysemanating from the x-ray source enter the x-ray receiving section, thex-ray receiving section generating an image representing intensities ofthe x-rays entering the x-ray receiving section; a computer coupled tothe x-ray receiving section; radiation sensors located in a path ofx-rays emitted from the x-ray source, the radiation sensors detecting abeginning and an end of a radiation cycle and transmitting the detectedbeginning and end of the radiation cycle to the computer; andcalibration markers for determining a projective transformation of theimage.
 6. The device of claim 5, wherein the calibration markers areused to correct image distortion.
 7. An x-ray imaging device comprising:a rotatable C-arm support having first and second ends, an x-ray sourcepositioned at the first end for initiating an imaging cycle bygenerating x-ray radiation; an x-ray receiving section positioned at thesecond end so that x-rays emanating from the x-ray source enter thex-ray receiving section, the x-ray receiving section generating an imagerepresenting the intensities of the x-rays entering the x-ray receivingsection; a calibration and tracking target; a tracking sensor fordetecting a position, in three-dimensional space, of the calibration andtracking target; a computer communicating with the x-ray receivingsection and the tracking sensor, the computer detecting motion of theC-arm based on changes in the position detected by the tracking sensor;a dynamic reference frame marker attached in a fixed position relativeto a patient, the tracking sensor detecting the position, inthree-dimensional space, of the dynamic reference frame marker; andmeans for detecting a beginning and an end of an imaging cycle andtransmitting indications of the detected beginning and end of theradiation cycle to the computer, and the computer determining that animage acquisition cycle is erroneous when the position of the trackingtarget moves with respect to the dynamic reference frame marker duringthe imaging cycle.
 8. A surgical instrument navigation systemcomprising; a computer processor; a tracking sensor for sensingthree-dimensional position information of a surgical instrument andtransmitting the position information to the computer processor; amemory coupled to the computer processor, the memory including computerinstructions that when executed by the computer processor cause theprocessor to generate an icon representing the surgical instrument andto overlay the icon on a pre-acquired x-ray image, the icon of thesurgical instrument representing the real-time position of the surgicalinstrument projected into the pre-acquired x-ray image and the iconbeing generated as a first representation when the surgical instrumentis positioned such that it is substantially viewable in the plane of thepre-acquired image and the icon being automatically generated as asecond representation when a distance in the pre-acquired image betweena tip and tail of the surgical instrument becomes smaller than apre-determined distance such that the surgical instrument issubstantially perpendicular to the plane of the pre-acquired image; anda display coupled to the processor for displaying the generated icon asone of either the first and second representations superimposed on thepre-acquired image.
 9. The computer system of claim 8, wherein the iconis superimposed on multiple pre-acquired images.
 10. The computer systemof claim 8, wherein the second representation includes first and secondcross-hair icons, the center of the first icon representing one end ofthe surgical instrument and the center of the second icon representingan opposite end of the surgical instrument.
 11. A surgical instrumentnavigation system comprising: a computer processor; a tracking sensorfor sensing three-dimensional position information of a surgicalinstrument and transmitting the position information to the computerprocessor; a memory coupled to the computer processor, the memoryincluding computer instructions that when executed by the computerprocessor cause the processor to generate an icon representing thesurgical instrument positioned in a pre-acquired image of a patient'sanatomy, the icon of the surgical instrument including a first portioncorresponding to an actual position of the surgical instrument and asecond portion corresponding to a current projection of the surgicalInstrument along a line given by a current trajectory of the surgicalinstrument where a length of the second portion is selectable by a userof the surgical instrument navigation system; and a display coupled tothe processor for displaying the generated icon superimposed on thepre-acquired image.
 12. The computer system of claim 11, wherein theicon is superimposed on multiple pre-acquired images.
 13. A surgicalinstrument navigation system comprising, a rotatable C-arm including anx-ray source and an x-ray receiving section for acquiring x-ray imagesof a patient, the C-arm being rotatable about one of a plurality ofmechanical axes; a computer processor coupled to the rotatable C-arm; amemory coupled to the computer processor, the memory storing the x-rayimages acquired by the rotatable C-arm and computer instructions thatwhen executed by the computer processor cause the computer processor togenerate a line representing a projection of a plane substantiallyparallel to one of the plurality of the mechanical axes of the C-arminto the x-ray image, the line enabling visual alignment of the one ofthe plurality of mechanical axes of the C-arm with an axis relatingcomplimentary x-ray images; and a display coupled to the processor fordisplaying at least a portion of the generated line superimposed on thex-ray image, wherein the line displayed on the display enables visualalignment of the C-arm with the complimentary axis before thecomplimentary x-ray image is acquired.
 14. The system of claim 13,further comprising: a tracking target attached to the rotatable C-arm;and a tracking sensor for detecting a position, in three-dimensionalspace, of the tracking target, wherein the computer calculates an amountof rotation of the C-arm based on changes in the position detected bythe tracking sensor.
 15. A system for defining a surgical plancomprising: an x-ray imaging device; a surgical instrument; a trackingsensor for detecting a position, in three-dimensional space, of thesurgical Instrument; a computer processor in communication with thetracking sensor for calculating a projection of a trajectory of thesurgical instrument a distance ahead of an actual location of thesurgical instrument; and a display coupled to the processor fordisplaying a pre-acquired x-ray image overlaid with an iconicrepresentation of the surgical instrument and a calculated projection ofthe trajectory of the surgical instrument, wherein a length of theprojection of the surgical instrument changes based upon an inputreceived by a user.
 16. A method of defining a surgical plan comprising:acquiring an image of a patient's anatomy; sensing three-dimensionalposition information of a surgical instrument; generating a graphicalicon representing the surgical instrument positioned in a pre-acquiredimage of the patient's anatomy, the icon of the surgical instrumentincluding a first portion corresponding to a position of the surgicalinstrument in space and a second portion corresponding to a currentprojection of the surgical instrument along a line given by a currenttrajectory of the surgical instrument; selecting a length of the secondportion corresponding to the current trajectory of the surgicalinstrument; and displaying the generated icon superimposed on thepre-acquired image.
 17. The method of claim 16, further comprisingfreezing at least the first or second portion of the iconicrepresentation of the surgical instrument in the x-ray image.
 18. Themethod of claim 16, wherein the icon is superimposed on multiplepre-acquired images.
 19. A method of representing a real-time positionof a surgical instrument in a pre-acquired x-ray image comprising:generating an icon of a surgical instrument and overlaying the icon onthe pre-acquired x-ray image, the icon of the surgical instrumentrepresenting the real-time position of the surgical instrument projectedinto the pre-acquired x-ray image; representing the icon as a firstrepresentation when the surgical instrument is positioned such that itis substantially viewable in a plane of the pre-acquired image; andautomatically representing the icon as a second representation when adistance in the pre-acquired image between a tip and a tail of thesurgical instrument becomes smaller than a pre-determined distance suchthat it is substantially perpendicular to, the plane of the pre-acquiredimage.
 20. A method of detecting an error in an x-ray processcomprising: generating cycles of x-ray radiation corresponding to animage acquisition cycle, the cycles being generated by an x-ray imagerand passing through calibration markers of a calibration and trackingtarget; generating an image of a patient's anatomy defined byintensities of the x-rays in the cycle of the x-ray radiation; detectinga beginning and an end of a radiation cycle; detecting a position of thecalibration and tracking target and the patient; and determining thatthe image acquisition cycle is erroneous when the position of thetracking target relative to the position of the patient moves betweenthe beginning and the end of the radiation cycle.
 21. A system ofdefining a surgical plan comprising: means for sensing three-dimensionalposition information of a surgical instrument; means for generating agraphical icon representing the surgical instrument positioned in apre-acquired image of a patient's anatomy, the icon of the surgicalinstrument including a first portion corresponding to a position of thesurgical instrument in space and a second portion corresponding to aprojection of the surgical instrument along a line given by a currenttrajectory of the surgical instrument; means for selecting a length ofthe second portion corresponding to the current trajectory of thesurgical instrument; and means for displaying the generated iconsuperimposed on the pre-acquired image.
 22. A system for representing areal-time position of a surgical instrument in a pre-acquired x-rayimage comprising: means for generating an icon of a surgical instrumentand overlaying the icon on the pre-acquired x-ray image, the icon of thesurgical instrument representing the real-time position of the surgicalinstrument projected into the pre-acquired x-ray image; means forrepresenting the icon as a first representation when the surgicalinstrument is positioned such that it is substantially viewable in aplane of the pre-acquired image; and means for representing the icon asa second representation when a distance in the pre-acquired imagebetween a tip and a tail of the surgical instrument becomes smaller thana predetermined distance such that it is substantially perpendicular tothe plane of the pre-acquired image.
 23. An x-ray imaging devicecomprising: means for generating cycles of x-ray radiation correspondingto an image acquisition cycle; means for generating an imagerepresenting intensities of the x-rays entering an x-ray receivingsection; means for detecting a beginning and an end of a radiation cycleand transmitting the detected beginning and end of the radiation cycleto a computer; a dynamic reference frame marker attached in a fixedposition relative to a patient; a calibration and tracking target; and atracking sensor for detecting a position, in three-dimensional space, ofthe dynamic reference frame marker and the calibration and trackingtarget; wherein the computer determines that an image acquisition cycleis erroneous when the position of the calibration and tracking targetmoves relative to the position of the dynamic reference frame during animage acquisition cycle.